This invention relates to a novel microfiber, to a nonwoven web comprising said microfiber, and to a method of preparing said web. More particularly, this invention relates to a microfiber which is prepared by meltblowing a liquid crystal polymer.
The technique of meltblowing is well known in the art and involves extruding a molten polymeric material into fine streams and attenuating the streams into very fine fibers by impinging a high velocity heated gas against the molten polymer that exits the extrusion die. The high velocity gas, usually air, may be maintained at an elevated temperature and serves to attenuate the molten resin to form fibers or, depending upon the degree of attenuation, microfibers having diameters less than the diameter of the capillaries of the die. As used herein, the term microfibers refers to fibers having a diameter of less than 150 .mu.m. The die melt temperature, i.e., the temperature of the polymer melt at the die, is generally about 80.degree.-100.degree. C. above the melting temperature of the resin. Meltblowing processes are described for example in Buntin et al. U.S. Pat. No. 3,978,185; Buntin U.S. Pat. No. 3,972,759; and McAmish et al. U.S. Pat. No. 4,622,259. These patent disclosures are hereby incorporated by reference.
The meltblowing process has been most commonly used for the meltblowing of polyolefins, such as polypropylene. Nonwoven webs made from polypropylene microfibers are soft and drapable and have been used as meltblown webs or as components in composite nonwoven fabrics. As is reported in Tappi, The Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, pages 74-77 (April 1973), other polymers which have been meltblown successfully include polyethylene, nylons, polyesters such as polyethylene terephthalate and poly(tetramethylene terephthalate). Various elastomeric polymers have also been meltblown to form nonwoven webs.
A class of high performance polymers has been developed which exhibit relatively high thermal stability, a high degree of resistance to chemicals and radiation, and relatively high tensile strength and which are based upon liquid crystal thermoplastic polymers. A particular class of such polymers which has received considerable recent interest are liquid crystal polyester polymers. These liquid crystal polyester polymers have been used as molding resins for high temperature stable products, in films, and to form fibers of conventional textile sizes by melt spinning. For example, U.S. Pat. No. 3,975,487 describes a process for spinning high modulus fibers from a liquid crystalline copolyester by meltspinning. Also, the formation of liquid crystalline (optically anisotropic) meltspun polyester fibers is disclosed in U.S. Pat. Nos. 4,370,466; 4,503,005 and 4,699,746. Published European Application EP 166,830 also describes the formation of fibers and webs from meltspun optically anisotropic polyester polymers.